In a conventional relative-angle sensor, such as that disclosed in, for example, JP-A No. 2000-351383, the rotation angle between the input and output shafts when the steering wheel is held at a neutral position is assumed to be during an operation to detect a rotation angle between the input and output shafts. Thereafter, the relative angle changes when the steering wheel is operated. A control unit cumulatively adds or subtracts changes the relative angle periodically to find the rotation angle between the input and output shafts (an absolute angle). The relative angle changes because the steering wheel is operated. The control unit comprises a non-volatile memory for storing the rotation angle between the input and output shafts, a control microcomputer for writing the rotation angle between the input and output shafts into the non-volatile memory, and a capacitor for supplying power to the non-volatile memory and the control microcomputer. The control microcomputer stores the rotation angle into the non-volatile memory only when the transmission-ratio-varying steering apparatus is stopped. At that time, a battery supplies power to the non-volatile memory and the control microcomputer. As the transmission-ratio-varying steering apparatus is operated again, the control unit reads out the rotation angle, which was between the input and output shafts at the time the transmission-ratio-varying steering apparatus was stopped, from the non-volatile memory, and controls an operation to drive an electric motor.
The transmission-ratio-varying steering apparatus has a power-supply circuit for clamping a voltage (of about 12V) from a power supply and a generator for supplementing power supplied by the power supply to a voltage (in the range 3.3 to 5V) of an operating range of the control unit. Thus, a clamped voltage in the operating range is supplied to the control unit.
Generally, when a heavy load is inadvertently disconnected due to, for example, a terminal loosened from the battery in the course of a travel motion of the vehicle, a surge voltage (of about 30V or higher) is generated by the generator. In this case, when such a surge voltage is supplied to the conventional power-supply circuit in the conventional configuration, a self protection function operates to cut off power being supplied to the control unit. For this reason, when the self-diagnosis function of the power-supply circuit operates, no power is supplied to the control unit, so that the function of the control unit ceases to work. Therefore, with the transmission-ratio-varying steering apparatus stopped, the control unit is not capable of storing a rotation angle between the input and output shafts into the non-volatile memory. As a result, the transmission-ratio-varying steering apparatus has a problem that, as the transmission-ratio-varying steering apparatus is operated again, the present rotation angle between the input and output shafts cannot be fetched from the non-volatile memory. Consequently, the control circuit is not capable of controlling an operation to drive an electric motor.
A power-supply circuit including measures to clamp a surge voltage generated by the generator to a voltage in an operating range of the control unit can avoid a cut-off of the power being supplied to the control unit. However, a power-supply circuit is generally implemented as an integrated circuit (IC). Thus, components having sizes larger than the conventional ones must be used in the IC of the power-supply circuit. This leads to an increase in size of the power-supply IC.